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FORVM`s Classical Numismatics Discussion Board  |  Numismatic and History Discussions  |  Ancient Coin Forum (Moderator: Danny S. Jones)  |  Topic: BLUE PATINA 0 Members and 1 Guest are viewing this topic. « previous next »
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Author Topic: BLUE PATINA  (Read 3522 times)
Julian_II
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« on: March 04, 2003, 12:04:47 pm »

An experienced well known numismatic here in Argentina, told me an interesting thing about the blue patina. One of the few ways if there are any else, of developing this patina is when the coin was left inside the mouth of a dead body, this starts the process that will generate the blue patina.
Very interesting, isn`t it. Smiley
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bruce61813
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« Reply #1 on: March 04, 2003, 01:04:29 pm »

The blue is Azurite. For a real heay duty explaination of this type of corrosion on coins see: http://aic.stanford.edu/jaic/articles/jaic31-03-007.html
A short form of this articles is" "Azurite indicates that the artifact corroded in the presence of elevated carbonate activity. Microbiological decomposition of organic material is one source of elevated carbonates."
 See also http://aic.stanford.edu/jaic/articles/jaic21-01-001.html
A decaying body would produce a lot of CO2, and little exposeure to water or air.

BOTMN
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Robert_Brenchley
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« Reply #2 on: March 04, 2003, 02:46:11 pm »

Azurite is a copper carbonate, so this wouldn't be surprising.
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Robert Brenchley

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Rugser
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« Reply #3 on: March 04, 2003, 04:11:19 pm »

The reported one from the Argentinian friend is a fable.

The 90%  of our coins they have stayed in mouth to the corpse ... have we the 90% of blue patina?
The truth is that the patina of the coins is the result of a strange combination.

1)Composition of the alloy.

2) Components of the contained salts in the ground (each ground has his salinity)

3) Degree of damp of the ground.

It is try the fact that terrestrial  in same  two equal coins give to the corpse they engage colour of different patina.....as tells is my experience.

ser
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Julian_II
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« Reply #4 on: March 04, 2003, 06:08:36 pm »

As said before this is something a friend told me. I don`t stand in any position with so little information yet Smiley
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bruce61813
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« Reply #5 on: March 04, 2003, 09:07:53 pm »

What Rugser said is very true, both Azurite and Malachite are copper carbonate. Chemists are not sure what the exact mechanism is that produces the blue form, and in some cases the blue will turn to green with a small shift the the chemical environment - ie, acidity, water or several other things - that is why I put up the URL's earlier.

Bruce
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Robert_Brenchley
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« Reply #6 on: March 05, 2003, 09:14:01 am »

Azurite = Cu3(CO3)2(OH)2

Malachite = Cu2(CO3)(OH)2

As you see, they're very similar chemical formulae. The are both made up of copper, carbon, oxygen and water in slightly different configurations. My background is actually in geology; both are typically found near the surface as copper ores react with water and oxygen; the latter being of course found in much greater amounts near to the surface as it's carried down dissolved in water. The copper in coins will react in exactly the same way. Both minerals occur in very similar conditions, and are often found together, though malachite is commoner.
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Robert Brenchley

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« Reply #7 on: March 05, 2003, 03:47:53 pm »

The very clayey ground, where  is scarce the oxygen,  returns coins with green clear patinas up to the  color celestial.
The ground much sand, with many salts of chlorine (chlorides)  returns coins color green-malachite.

ser
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